The long-term objective of this research is to understand the regulation of ovarian function at the tissue level, by defining the mechanisms which permit one follicle to complete the process of follicular development and ovulate, while relegating others to atresia, before development is completed. The experiments in this proposal focus on the role of FSH in the selection process. The specific aims of this project are: 1) To define the mechanism by which elevated FSH recruits follicles by monitoring rates of granulosa cell proliferation, incidence of atresia, and evidence of cell differentiation following manipulation of endogenous FSH concentrations. 2) To characterize the feedback relationship between FSH and early antral follicles during the selection process on estrus by a) altering the number of early antral follicles or the concentration of FSH in the circulation on estrus and determining if these interventions prevent the appearance of atretic follicles in the intact rat on metestrus, and b) comparing the selection process in intact rats with that in log-term hemicastrate rats, where selection occurs without atresia. 3) To test the hypothesis that changes in the composition of the basement membrane are involved in the mechanisms of atresia by using immunohistochemical techniques to examine changes in the molecular composition of the basal lamina during the selection of early antral follicles on estrus. 4) To determine the mechanism by which the corpus luteum of the mentrual cycle inhibits follicular development in the rhesus monkey, by monitoring proliferation rates of granulosa cells and the incidence of atresia following luteectomy in different size classes of follicles.